1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses used in printers, copying machines, and facsimile machines, and more particularly, to an image forming apparatus including a nonmagnetic single component developing device.
2. Description of the Related Art
As an image forming apparatus using a single component developer, a so-called xe2x80x9ccontact developing devicexe2x80x9d in which an image bearing member (also referred to as a photosensitive drum) is brought into contact with a developer bearing member (also referred to as a development roller) has been disclosed, for example, in Japanese Patent Publication No. 2-26224 and Japanese Patent Laid-Open No. 3-261978.
FIG. 5 is a schematic diagram showing an example of a conventional image forming apparatus in which images are formed by a contact developing device using a nonmagnetic single component developer (hereinafter, a developer is also referred to as toner). As shown in FIG. 5, in such an image forming apparatus, there are disposed in the periphery of a photosensitive drum 100, which is an image bearing member rotating in the X direction in the drawing, a charging roller 101 as a primary charging device, an exposure unit 102 as an electrostatic latent image forming device, a developer unit 103 as a developing device, a transfer roller 104 as a transfer device, and a cleaning device 105.
The developer unit 103 includes a development roller 107 which performs development while rotating in the Y direction in the drawing, a feed roller 108 as a toner-feeding means which feeds toner Txe2x80x2 for nonmagnetic single component development to the development roller 107 while rotating in the Z direction in the drawing, a development blade 110 which is a toner-regulating means for regulating the amount of the toner Txe2x80x2 coated on the development roller 107 as well as the amount of charge, a stirring member 109 for feeding the toner Txe2x80x2 to the feed roller 108 and stirring the toner Txe2x80x2, etc.
In the contact developing device using the rigid photosensitive drum 100 which is brought into contact with the development roller 107 so as to perform development, the development roller 107 is preferably provided with an elastic body.
Conventionally, as the development roller 107 provided with the elastic body, an elastic development roller with a so-called xe2x80x9csingle solid layerxe2x80x9d is used in which a silicone rubber or a nitrile-butadiene rubber (NBR) is formed on a metallic core bar. As the development blade 110, a development blade is generally used formed of a thin plate composed of a stainless steel or the like to which a rubber member composed of a polyurethane rubber or the like is attached at the section abutting on the development roller 107.
Next, the image formation operation of the image forming apparatus will be described.
In response to a print signal from the outside, the photosensitive drum 100 starts to rotate in the X direction. First, the photosensitive drum 100 is uniformly charged by the charging roller 101. Next, an electrostatic latent image is formed on the photosensitive drum 100 due to exposure by the exposure unit 102, and the electrostatic latent image reaches the section which is in contact with the developer unit 103 when the photosensitive drum 100 is rotated.
In connection with the above operation, the developer unit 103 performs an operation described below.
The toner Txe2x80x2 stirred by the stirring member 109 is fed onto the development roller 107 due to sliding friction between the development roller 107 rotating in the Y direction and the feed roller 108 rotating in the Z direction. A predetermined amount of charge is applied to the toner Txe2x80x2 on the development roller 107 and the amount of the toner Txe2x80x2 is regulated by the development blade 110, and thus the toner is born on the development roller 107.
When the toner borne on the development roller 107 reaches the section which is in contact with the photosensitive drum 100, i.e., a development section, a developing bias is applied to the development roller 107 by a power source (not shown in the drawing), and thus the electrostatic latent image formed on the photosensitive drum 100 is developed by the toner Txe2x80x2 borne on the development roller 107 so as to become visible. The toner which is not used for development and which remains on the surface of the development roller 107 is recovered by the developer unit 103 via the feed roller 108.
The toner on the photosensitive drum 100 reaches the section opposite to the transfer roller 104 due to the rotation of the photosensitive drum 100, and is transferred to a sheet of transfer paper Pxe2x80x2. The toner Txe2x80x2 on the transfer paper Pxe2x80x2 is subjected to thermofusion fixing by the fixing device 106, and thus a permanent image is produced.
The remaining toner Txe2x80x2 which is not transferred to the transfer paper Pxe2x80x2 is recovered by the cleaning device 105.
By repeating the operation described above, the image formation is repeated.
However, there are problems as described below depending on the characteristics of the development roller.
(1) Development Roller Having Electron Conduction System
When a silicone rubber is used as the development roller, a predetermined resistance of the development roller is obtained by dispersing carbon particles or metal particles into the silicone rubber. Such a conduction mode in which a predetermined resistance is obtained by dispersed particles is generally referred to as an electron conduction system. However, when a development roller having the electron conduction system with a low resistance of approximately 1xc3x97104 xcexa9 is used, development characteristics become binary and it is not possible to obtain desired tone characteristics. Although the binary development characteristics are advantageous for text images (i.e., linear images), they are disadvantageous for photographic images (i.e., picture images) because it is impossible to reproduce images with highlights.
As described above, in the development roller having the electron conduction system, it is difficult to obtain satisfactory tone reproduction in output images.
On the other hand, in a development roller with a high resistance of 1xc3x97106 xcexa9 or more, tone characteristics with gentle gradation can be obtained. However, if the resistance is increased in the development roller having the electron conduction system, for reasons which are not yet clear, when the longitudinal image width is changed, the density of the developed image varies even for the same latent image condition, which is disadvantageous. Such a phenomenon is noticeable, particularly in an image with highlights having a low density. Although a desired density can be obtained when the image width is narrow, it is not possible to obtain the desired density when the image width is wide. That is, when an image with highlights is output in the full longitudinal width, it is not possible to obtain a desired density.
Additionally, with respect to the electron conduction system, when an applied voltage is low, the resistance of the elastic body is increased, and in an extreme state, the resistance may vary by 3 orders of magnitude or more, resulting in difficulty in forming a desired image.
(2) Development Roller Having Ionic Conduction System
In a development roller, by adding an ion-conductive agent to an NBR, a polyurethane rubber, or the like so that the material itself is ionized to form a conductive path, a predetermined resistance of the development roller is obtained. Such a conduction system based on the movement of ions is generally referred to as an ionic conduction system. When a development roller having the ionic conduction system is used, in a manner differing from that of the development roller having the electron conduction system, a substantially constant resistance is obtained in response to an applied voltage. Therefore, the ionic conduction system is advantageous over the electron conduction system in the case of the formation of an image having gradation such as a picture image.
However, in the ionic conduction system, since a current path is generated by the ionization of the material, the degree of ionization differs depending on the environment. Under high temperature and high humidity conditions, the resistance of the development roller is decreased, while under low temperature and low humidity conditions, the resistance of the development roller is increased.
Consequently, since the amount of current flowing into the development roller differs depending on the change in environment, under high temperature and high humidity conditions, the density of the image is increased and the overall image becomes dark. Under low temperature and low humidity conditions, the density of the image is decreased and the overall image becomes too light.
In this way, in the development roller having the ionic conduction system, the density of the image varies with environmental conditions, and thus it is difficult to obtain a stable image density.
As described above, it is difficult to obtain desired gradation in the development roller with a single layer having the electron conduction system or the ionic conduction system.
Furthermore, recently, for space-saving purposes, various types of image forming apparatuses employing a cleanerless system which does not include a cleaning member, which mainly cleans a developer remaining on the surface of a photosensitive drum after the transfer process, have been proposed. As one such image forming apparatus, an apparatus employing a xe2x80x9cdevelopment-and-cleaningxe2x80x9d system has been proposed in which a charging roller is brought into contact with a photosensitive drum by means of conductive particles to charge the photosensitive drum, a development roller and the photosensitive drum are disposed so as to abut each other, and in the development process, development is performed using a developer containing the conductive particles and the photosensitive drum is also cleaned.
However, when the above-mentioned image forming apparatus is used under high temperature and high humidity conditions, an electrostatic latent image is sometimes disturbed at the abutting section between the development roller and the photosensitive drum. Such a phenomenon occurs because the development roller to which a voltage is applied abuts on the photosensitive roller, and therefore charge injection occurs by means of the conductive particles at the abutting section between the photosensitive drum and the development roller, and the potential in the exposed section is partially changed. Consequently, in an image with highlights, etc., unevenness may occur in the image and a desired image density may not be obtainable.
Additionally, although Japanese Patent Laid-Open No. 8-73660 discloses a conductive roll provided with a conductive elastic layer, which contains carbon black and an ion-conductive agent, on the surface, it does not describe that the conductive roll is used as a development roller for contact-charging.
The present invention has been made to overcome the problems described above. It is an object of the present invention to provide an image forming apparatus in which the density is not affected by the image width and an image having superior tone characteristics can be produced even under high temperature and high humidity or low temperature and low humidity conditions. It is another object of the present invention to provide an image forming apparatus in which unevenness in images does not occur and a satisfactory image can be formed even in a system where an image bearing member and a developer bearing member are in contact with each other. It is another object of the present invention to provide an image forming apparatus which can form a high-resolution image even when the xe2x80x9cdevelopment-and-cleaningxe2x80x9d system is employed.
In accordance with the present invention, an image forming apparatus includes an image bearing member for bearing an electrostatic latent image, a charging device for charging the surface of the image bearing member, an electrostatic latent image forming device for forming the electrostatic latent image on the surface of the image bearing member charged by the charging device, a developing device, which includes a developer bearing member for bearing a single component developer, for performing development by bringing the single component developer into contact with the electrostatic latent image to form a visible image, a transfer device for electrostatically transferring the visible image to a transfer member, and a fixing device for fixing the electrostatically transferred visible image on the transfer member. The developer bearing member includes at least a conductive core bar, an electron-conductive layer composed of an elastic body, and an ion-conductive layer.
Further objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.